1. Field of the Invention
This invention pertains generally to MEMS capacitors, and more particularly to a MEMS tunable capacitor that employs an angular vertical comb drive.
2. Description of Related Art
Research in wireless communication devices has exploded exponentially with the increase in demand for smaller, cheaper, and more powerful portable cellular handsets. In the past p-n junctions (varactors) have been traditionally used in wireless communications as tunable capacitors for frequency control, filtering, and synthesizers. However, their drawbacks have led to research in Micro-Electro-Mechanical Systems (MEMS) as a possible alternative technology in RF and microwave systems.
Generally, the use of MEMS-based tunable capacitors can substantially increase the tuning range (>30%), provide higher Q's, and can tolerate higher voltage swings while consuming less power. MEMS devices therefore provide inherent performance advantages over conventional solid-state varactors. Additionally, as MEMS tunable capacitors exhibit a low mechanical resonance frequency, harmonic distortion is no longer a concern. If a large tuning capability can be provided, a MEMS tunable capacitor can be integrated in an array of military and commercial applications such as wireless systems.
MEMS tunable capacitors (varactors) can replace p-n junctions as frequency controllers to generate and filter transmitting and receiving signals. They can also be used as phase shifters in phased-array antennae. Although MEMS capacitors can provide superior performance over conventional devices, they are not expected to achieve widespread commercial use unless fabrication can be made more cost-effective.
The capacitance of a MEMS varactor can be varied by either moving the dielectric between the conductors, or changing the spacing or overlap area between the conductors. Several actuation mechanisms have been reported, including thermal, piezoelectric, or electrostatic actuation. Electrostatically-driven varactors have received the most attention because of their low power consumption, fast response time, and ease of control due to hysteresis-free tuning. The simplest type of tunable capacitor consists of a pair of parallel plate conductors whose spacing is varied by applying a dc voltage. This is often called a gap-closing actuator.
Most of the attention has been directed toward MEMS tunable capacitors employing gap-closing electrostatic actuation between two parallel plates. This design offers relatively short response times with a low power dissipation and little or no heat generation. However, these gap-closing actuators also suffer from some fundamental limitations. For example, these devices exhibit highly nonlinear actuation and also exhibit a pull-in phenomenon at about one third of the gap distance between conductors, which limits the tuning ratio to about 1.5:1 (50%). There have been several attempts to overcome this theoretical limit via modifications to the gap-closing design. Nevertheless, the tuning ratios achieved are still very modest.
There have been attempts to overcome this theoretical limit by employing different gap spacings for the actuator and the capacitor, or using a push-pull geometry. One researcher achieved a tuning range of 69% while another group was able to obtain an 87% change in capacitance with this approach. However, these modifications demonstrate only moderate increases. Separation of the sensing electrodes and actuating electrodes on these parallel plate capacitors has yielded a tuning range to 600%.
In recent years, lateral comb drive actuators are receiving increasing attention as a superior alternative to gap-closing actuators. The use of lateral comb drive actuators has made it possible to achieve a tuning ratio of 8.4 to 1 with low operating voltages. While lateral comb drives do not suffer from pull-in, the capacitance tuning relies on the lateral motion of the movable fingers. Hence, the tuning ratio is limited by the maximum separation of these fingers and their overall lengths, which has the negative effect of increasing overall device size.
FIG. 1 depicts an example of a conventional MEMS tunable capacitor with laterally interdigitating fingers which provide a maximum capacitive tuning range of about 740% (8.4:1). The tuning occurs by electrostatically displacing the movable comb, whose fingers are laterally displaced in relation to the fingers of the stationary comb.
Despite the advantages of MEMS tunable capacitors, a fundamental limit exists for lateral comb drive devices; the lengths of the sensing fingers must be shorter than the total displacement of the comb drives. Thus optimum tuning and maximum capacitance value require a large device area and/or comb finger thickness. The larger device size reduces the ability to effectively integrate the MEMS device into microwave systems.
Therefore, a need exists for a MEMS tunable capacitor configuration that has a wide tuning range, requires less physical circuit space than conventional MEMS tunable capacitors, and which can be readily fabricated. The present invention satisfies those needs, as well as others, and overcomes inherent deficiencies in current MEMS tunable capacitor designs.